This chapter is from the book
This chapter is from the book
This chapter is from the book
Computed Properties
It is frequently useful to find a vector’s length or magnitude. You could do this by adding a function returning a Double:
struct Vector {
...
func length() -> Double {
return sqrt(x*x + y*y)
}
}
However, it is much more natural to think of this as a read-only property. In Swift the general term for this is computed property, which is in contrast to the stored properties you have been using so far. A read-only computed property version of length would look like this:
struct Vector {
...
var length: Double {
get {
return sqrt(x*x + y*y)
}
}
}
This read-only computed property pattern (called a “getter”) is so common, in fact, that Swift provides a shorthand means of expressing it. Add this to Vector:
struct Vector {
...
var length: Double {
return sqrt(x*x + y*y)
}
}
At other times it is useful to have a getter and setter for a computed property. This tends to be used to alias other properties or to transform a value before it is used elsewhere. For example, you could abstract the setting of the textField from the RandomPassword with a computed property:
class MainWindowController: NSWindowController {
@IBOutlet weak var textField: NSTextField!
var generatedPassword: String {
set {
textField.stringValue = newValue
}
get {
return textField.stringValue
}
}
...
@IBAction func generatePassword(sender: AnyObject) {
let length = 8
generatedPassword = generateRandomString(length)
}
}
Computed properties do not have any storage associated with them. If you need to store a value, you must create a separate stored property for it.